Consumers which need a high level of D.C. supply voltage can be connected to a supplying battery. For high levels of output voltage, batteries consist of many cells which are connected in series. It may be necessary to connect one potential of the battery to the protective earth potential, e.g. for safety reasons. Separating the battery into two serial portions and connecting the such created central point to protective earth may be useful to reduce the insulation effort or voltage stress which occurs between any of the battery poles and any parts which are connected to protective earth potential.
In order to bring batteries to a desired state of charge, commercially available battery charging devices, also called chargers, may be used. Connecting one charger to a battery which consists of two serial portions which are centrally earthed, may lead to undesired high voltage in case of a single insulation fault, e.g. an insulation breakdown between one of the battery poles and a part which is connected to protective earth potential. This can be avoided, if a separate charger is used for any of the battery portions.
Non-symmetry charging of portions of batteries may lead to different states of charge of each of the portions of the battery which is not desired, resulting in unbalanced aging and capacity loss. Prevention of such different states of charge requires very high accuracy of the charging current measurements and state of charge measurements, for instance by a sophisticated and complex charge control. Available charging devices operate and balance parts or portions of the battery by, for instance, a so-called current regulation mode or by a voltage regulation mode.
However, modern lithium ion batteries have a rather flat charging curve. This means that the battery voltage hardly depends on state of charge for a wide range of the state of charge. As a consequence, the voltage regulation mode does not work properly in the flat area of the charging curve, because the charge state cannot be detected by a simple voltage measurement. Consequently, current regulation mode is preferably applied in the flat area of the charging curve.
International patent application publication WO2009145709A1 describes a control circuit for battery unbalance mitigation.
Japanese patent application publication JP2004194410A describes a power supply device for a vehicle.
USA patent publication U.S. Pat. No. 6,271,645B1 describes a method and circuit for balancing energy levels among first and second battery groups within a battery pack.